Seeing the Other Side
by YamiAngelOfTheNight
Summary: Takes place after the Pacifist Run. Sans thought that everything could be normal. Living in the Underground wasn't so bad when there was access to the upper world. However, he never expected a strange new enemy as well as an old face to present itself. And their single target? Frisk. WARNING.Possible spoilers to the Genocide run and the game in general. Read at your own risk.
1. Chapter 1

**Hey everybody. So, I just got to the Sans fight in the Genocide route (though I doubt I'll complete it. I don't think I have it in me to kill Sans). This story came to me one day, about what would happen if Sans met Genocide Sans. Hence this story. Hopefully you guys all enjoy.**

* * *

 _"Please...I'm sorry. I didn't want to!"_ _Sans looked down, feeling nothing but anger towards the boy knelt before him. He was coughing and hiccuping, his crying having got the better of him. The little boy looked up at him, one hand over his chest. His soul floated before him, the red color a stark contrast to the darkened room. The boy looked up, eyes sliding open for the first time since he'd known them. Dark eyes slowly faded to red, the sad expression turning more twisted. "This was your one chance to save him," the boy taunted, voice distorted._

 _"I know," he said, raising a hand. The bones that followed his command hovered behind him, all pointing towards the boy. The boy frowned._

 _"What?" he asked, "you'd kill this child?" Sans chuckled._

 _"I told you before didn't I?" he said, "That you'd have a bad time. Now goodbye...Frisk..."_

* * *

Sans' eyes snapped open, left eye glowing with energy. He sat up, taking a deep breath to steady himself. _'That dream again,'_ he thought. He shook his head, looking around. _'I must have fallen asleep on the couch again.'_ He stood, calming down the energy flaming around him and heading for the stairs. _'Better check on the kids. Toriel would be livid if something happened to them.'_ He pushed open the door to his brother's room, peering inside. Papyrus was lying in the center of the bed, arms wrapped around the children on either side of him. Frisk was lying on his back, sprawled out with limbs everywhere. Asriel was curled into Papyrus' side, and Sans smiled recalling how one of the human souls had stayed behind to give the little monster another chance. He chuckled, turning and leaving.

Halfway down the stairs he paused, energy flaring to life again. Another scene passed before his eyes. Frisk kneeling on the ground, staring up at him.

 _"You plan on coming back again...don't you. Fine. But I'll be here to knock you down again kid. Remember that."_ Frisk's smile, even as blood pooled around him.

 _"I don-*cough*...don't plan on coming back. Just...Sans...I-I never-*cough cough*...wanted to hurt him. Never...Papyrus..."_

The vision faded, leaving the skeleton slightly dizzy. _'Papyrus?'_ he thought, _'Why would I have a thought like that? Of Frisk...killing Papyrus.'_ He shook his head, feeling his energy settle down. He started down the stairs again, planning to head for the couch again and get some more sleep.

"Sans?" he stopped, looking back. Frisk stood halfway up the stairs, watching him with tired eyes. "Is something wrong?" The skeleton shook his head, smiling.

"Everything is fine kid," he said, "so you should go back to sleep." He turned away, only to feel a hand grip his. He looked back, seeing Frisk. Now that they kids had grown a bit, each around the age of ten or so, Sans no longer stood above them. He'd grown a bit himself, but now was only an inch or so taller than the other. Frisk looked at him, silent. Sans sighed. "Fine kid. but just for tonight." Frisk giggled, flashing a smile.

"Okay Sans," Frisk said. The skeleton chuckled and laid down again, allowing the boy to curl up beside him. He shut his eyes, falling back into sleep.

* * *

 _'Well well...so peaceful he is. Too bad it's going to be his last day. Goodbye human child. It's time to meet your fate.'_ He readied his attack, eye flashing.

* * *

Sans snapped awake, raising a wall of bones on instinct. Frisk woke as another attack slammed against it, loud cracking noises sounding. "S-Sans?" Frisk said, "What's happening?"

"Don't worry kid," Sans said, "go upstairs and wake Pap." He created a safe path for the human to run, footsteps echoing up the stairs. Sans let his wall fade, finally getting a look at their attacker. What he saw surprised him.

"Ready to have a bad time?" the other asked with a grin. Sans glared, energy flaring to life more than ever before. He could hear Papyrus coming downstairs, calling out to him.

"SANS?" Papyrus asked, "WHY ARE THERE TWO OF YOU?" Sans ignored him, eyeing the copy of himself standing a few feet away. His signature jacket was white, and Papyrus' scarf was wrapped around the copy's neck. A long, bloodied slash ran from the double's right shoulder to his left hip. At the place where Sans' eye normally glowed blue, the double's eye glowed red.

"Who are you?" Sans demanded. The double shifted, cocking his head.

"I'm you," the double said, "from another timeline. A timeline where he-" The double pointed towards Frisk, "ruined everything."

"Frisk could never hurt anyone!" Asriel yelled. The double shot Asriel a glare, raising a hand. Asriel's soul blinked into existence before him, suddenly turning dark blue. The double lifted him up, ignoring his shocked cry.

"In this timeline...you might see him as innocent. A _**pacifist.**_ But in my timeline...he became a killer. I come from a timeline when your _**friend**_ committed a complete genocide. Every monster. Dead." He tossed Asriel aside, giving a viscous grin as he looked back at Sans. "You're me from this timeline. Surely you've seen that potential. That...with one wrong move...this _**human**_...could become a killer."

"SANS!" Papyrus said, "WHAT IS THIS CLONE TALKING ABOUT?" Sans stayed silent, watching his double closely. He felt someone grab his sleeve, looking down to see Frisk half hiding behind him.

"This child hasn't done anything wrong," Sans said, "I dunno what the human in your timeline did, but Frisk here couldn't hurt anyone." The double laughed, left eye flaring while his right eye went dark.

"You wanna know what he's done. What he has the _**potential**_ to do? In your timeline, you still have your friends. You still have your _**brother.**_ In my timeline..." He closed his eyes, and when they opened again they were nothing but black holes. "everybody is dead..." Sans frowned, energy blazing around him as the double did the same. "Normally...I'd kill you for standing in my way. However, killing myself doesn't seem like a good way to go. Besides, killing another me in another timeline? There's no telling what would happen then." The other skeleton chuckled. "Welp. I'll be seeing you. Just a warning though kid. If I see you, then you're a dead human."

"F-Frisk!" Asriel said, going over to cling to the human, "What's going on?"

"Sans?" Frisk asked. The skeleton sighed.

"Don't worry kid," he said, "Everything's gonna be fine."

"THAT'S RIGHT. SANS WILL FIGURE IT OUT. HE'S JUST THAT COOL!" Yet for once, despite his brother's confidence, the skeleton wasn't entirely sure he believed his own words.

* * *

 **So, lemme explain my design for Sans. He's one of those people who I feel like is extremely powerful, but he can't beat everything (As evident by how the player can beat him in the Genocide Run and Flowey can overpower him in the Pacifist Run). I figure he knows what battles he can and can't handle, and with Genocide Sans having nothing to lose he's got the advantage.**

 **Anyway, thanks for reading. Please leave a comment. It'll help a lot to develop the story and my writing as a whole. See ya next time.**


	2. Chapter 2

**Hey there guys. Sorry it's been so long. I kind of got writers block on where to go with this but now I know. Hopefully you'll enjoy this chapter so read on you lovely people. Couldn't do this without you.**

* * *

"Oh my! What do you plan on doing Sans?" The small skeleton looked at the group assembled with him at Grillby's. Undyne was clicking her nails against the table, teeth grit. Alphys sat next to her, clawed hands over her mouth in shock. Toriel was watching him carefully, glancing over to where Frisk and Asriel were playing with Papyrus and Mettaton.

"This... _ **Genocide Sans**_..." Undyne began, "what do we do?"

" _ **We**_ don't do anything," Sans said, "You just keep the kid safe. I'll take care of this double." Undyne snorted, looked annoyed.

"Yeah," she said, "cause that'll end well." Alphys and Toriel stared at her. "Oh come on. It's obvious he won't win. They might be evenly matched power wise, but from the sounds of things this Genocide Sans has nothing more to lose. That kind of desperation is a dangerous thing. On your own you don't stand a damn chance. I say send the kid in. Hell for this other Sans to exist it means Frisk is stronger than you right?" Sans' eyes sockets turned down into an almost frustrated look, but before the skeleton could talk Frisk climbed up on the seat next to him.

"Children?" Toriel questioned gently. Asriel pulled himself into the seat beside her, snuggling into her side. Mettaton and Papyrus were right behind them, sheepish looks on their faces. "you were supposed to wait for us to get you?"

"But we got tired mom," Asriel muttered, shutting his eyes, "Despite popular belief we can't keep going forever." Sans smirked at the monster child's words before a hand touched his arm. He looked at the boy beside him, quirking an eyesocket at him.

"Someone's watching us...outside..." Frisk said, so soft that Sans barely even heard him. He glanced up casually, stiffening when he saw his double at the door. Genocide Sans smirked at him, cocking his head and dragging a single, bony finger across his neck. When the other winked at him Sans felt his energy flare to life, left eye lighting up in a puff of blue flame.

"SANS?" He paused, looking at his brother. "IS SOMETHING WRONG?" When Sans glanced back to the door, Genocide Sans was gone. He shook his head, letting the energy die out.

"Nah. I must be bone-tired or somethin." He only half heard his brothers annoyed groan, glancing back to the window. The other was gone now, leaving nothing but the flurry of snow outside.

* * *

"Undyne?" The guard turned, coming face to face with the woman in her doorway. Alphas looked concerned, worry clouding her entire presence. "You don't..." She trailed off, shifting with discomfort.

"What's wrong Alphy?" She asked. The smaller woman sighed.

"It's just...you d-don't actually...think we should s-send Frisk after this other Sans do you?" Undyne blinked, twirling her spear.

"Well what else are we gonna do?" She asked, "Clearly this double has been beat by Frisk before, so why not this time?"

"But Undyne..." Alphas said, sounding frustrated.

"What?" The fish asked.

"In order for this other Sans to be the way he is..." Alphas started, turning away, "It means that...F-Frisk...killed him. And I-if this Genodice Sans was telling the truth then...he killed all of us as well." Undyne sighed, setting her spear down and approaching the other. Alphas was basically shaking, and Undyne assumed she was probably crying.

"Hey," she said, awkwardly hugging the other, "don't worry about it okay?"

"I d-don't even want to t-think about Frisk doing something like t-that. And if that double is right, t-then Frisk k-killed all of us too."

"Ngah...just forget what I said," Undyne told her, "it was a heat of the moment thing. Just pisses me off that the bone-head won't let us help." Alphas nodded in understanding.

"D-don't be too hard on Sans," the reptilian monster said, "he's doing his best. He's had to make some tough calls."

"Yeah, yeah," Undyne said, resting her chin on Alphys' head, "Still wanna bash his face in sometimes. Going at Genocide Sans would let me do just that."

"At least you aren't going at him f-for real," she said with a smile.

"Nah...Pap would never forgive me." Alphys sighed, leaning back against the other.

"Sans does whatever he can for Papyrus," Alphys said, "just...remember that...okay?"

"Uh...yeah. Sure Alphy. Whatever you say." She stepped back, stretching. "Gotta go. Pap is waiting for his training."

"H-have fun." Undyne flashed a smile and left, grabbing her spear as she did so. Once shed left her house, however, her smirk dropped. She sighed, running a hand through her hair.

"Trust me Alphy," she said, hanging her head, "I don't wanna send Frisk into battle either but...I'm not seeing another option here."

* * *

As Undyne left, Alphys frowned and leaned against the wall with a sigh. "This isn't good," she murmured, rubbing her hands over her face, "Sans...I know that you want your past to stay gone but...things are getting out of hand." She slid down the wall, curling into herself a bit. _'Undyne wants to use Frisk as a game piece. She thinks it's impossible to win this any other way.'_ Burying her face in her hands, the small monster trembled. "Don't let this happen Sans," she murmured, "don't sacrifice Frisk just to keep your secret."

She sat there for a while before releasing a breath and standing. _'No...'_ She thought, _'I can't just sit here and hope. I'd better go talk to him.'_ She bit her lip, thinking back to the last time she'd brought it up. The anger that had emanated from his form. The hatred when he retaliated with her own secret. _'But my secret is out this time,'_ she reminded herself, _'I've got to convince him. He's got everything to lose...especially if this Genocide Sans has the same secret.'_ She nodded, building up what confidence she had. _'I can do this. No...I have to. I refuse to stand by and watch someone get thrown to their death.'_

* * *

 **So there you have it. Sans' past is always fun to play with, but here's my challenge to you guys. What do you think he's hiding? I'd love to hear your thought so feel free to review or send me a PM. Anyway, thanks for reading. I'm off now to play catch up with my writing but I'll see you all in the next chap. Bye for now!**


	3. Chapter 3

**Hey there everybody. Here's a brand new chapter for you. I won't be revealing Sans' secret yet, but soon enough. Hope you all enjoy!**

* * *

"S-Sans?" She watched as the skeleton looked up, his grin widening.

"Something up Alphy?" he asked, saluting with his bottle of ketchup, "You look like ya got a bone to pick with me." She shifted, glancing around.

"Can we t-talk?" she asked before adding, "In private?" His grin faded and he turned back to Grillby.

"Put it on my tab Grillby," he said, "I've got something to take care of." The fire monster nodded, simply taking his bottle and walking off to empty it. Sans hopped down from his chair, walking past her. "Come on. I've got some place we can talk." She nodded, hurrying after him as fast as she could. He led her away from Snowdin, down to some cliff face she'd never been to. Then again, she hadn't been to much of Snowdin and its surrounding area. When he stopped she waited, unsure what to say. "So what's up?" he asked, though his voice betrayed is understanding of the situation. She fought for words before it simply spilled out.

"You can't let Frisk fight him!" She blurted. Sans didn't move. Didn't turn to face her.

"I didn't plan on it," he said, "but ya gotta admit. Kinda seems like our last option huh?"

"That's not t-true!" Alphys said, "You _**know**_ we have another option."

"Oh?" the skeleton asked. Growing frustrated, she clenched her claws.

"You k-know what I m-mean Sans!" She shouted, starting to get desperate, "I k-know you never wanted anyone to know b-but-" She was cut off by the sudden appearance of Sans' Gaster Blasters. The two dragon like skulls hovered beside him, fangs bared more than usual. He turned towards her slowly, left eye blazing with energy while the right was nothing more than a black hole.

"I thought I said not to speak about that," he said, voice menacing, "These Gaster Blasters are all the power you get from me. Nothing more."

"But-" Alphys started. One of the Gaster Blasters roared, snarling dangerously and making her jump. Sans calmed it with a simple raise of his hand.

"I refuse to use that power," Sans said lowly, frowning, "and I have no doubt that my copy will feel the same. Leave it be Alphys." She blinked, and he was gone. The only sign of his presence were the footprints pressed into the snow. She sighed, hanging her head.

 _'Well that went about as well as I expected,'_ she thought, _'Guess there's nothing I can do about it though. I've never seen him so serious...'_ She shook her head, resolving herself to retreating to the lab and thinking of a new solution.

* * *

"SANS! WHERE HAVE YOU BEEN!"

"I told ya Pap," he said, "I went to Grillby's."

"OH...RIGHT..." He chuckled at his brother's forgetfulness, being cut out of his thoughts by Frisk's excited giggles. The human ran past him, and he turned just in time to see Frisk wrap his arms around the Gaster Blaster's snout. He turned back to Toriel's laugh. She was watching Frisk fondly, Asriel hiding behind her.

"He truly loves those things of yours," she commented. He chuckled, nodding.

"Yeah," he said, looking back to the kid. The Gaster Blaster's mouth was open, as if it was trying to smile. Yet something about the scene felt off. He scanned them both, frowning. Everything looked normal...except one thing. The Gaster Blaster's eyes. Instead of the blue they were meant to be, they were replaced by bloody red. "That thing ain't mine," he said in shock. Toriel gasped.

"Frisk! Get away from it!" Frisk looked back, confused. His arms dropped as the Gaster Blaster reared back, energy filling its maw. Before any of them could react it fired with a loud bang, energy engulfing the human child within seconds. Asriel gave a scream, and Toriel easily swept up her child into her arms. Sans was forced to look away from the blast, dust and rock debris filling the air.

"How easy that was," a mocking voice said, "Would've thought that'd be a skele-ton harder...to...do?" Sans looked back, the dust settling. Frisk stood before the Gaster Blaster, completely unhurt. He giggled, patting the Gaster Blaster happily.

"That's right," Sans said, meeting his double's gaze, "the Gaster Blasters can't hit someone with a pure heart." Genocide Sans glared, left eye blazing red.

"Maybe so," he said, "but it can still bite." He raised a hand, signaling the creature. It roared loudly, baring its fangs. Toriel stepped forward, but Sans raised a hand to stop her. Frisk smiled warmly at the Gaster Blaster, opening his arms.

"Look at you," Frisk giggled, "acting all big and intimidating. We both know you're not gonna hurt me." The Gaster Blaster gave a small purring noise, nuzzling its nose into Frisk's chest.

"I suggest you go back to wherever you came from," Sans told his double, "or else you're gonna have a real bad time." The other frowned, eye sockets going dark.

"Thanks to this little human I have nowhere left to go back to," Genocide Sans said, "so don't think this is over yet." Without another word he was gone, taking the Gaster Blaster with him. Toriel ran forward to scoop Frisk into up into her free arm, and Asriel squirmed to hug the human.

"What are we going to do Sans?" Toriel asked, "I refuse to lose my family again."

"Don't worry," he said, "There's a way to get rid of him." Toriel nodded, turning and taking the children inside. Sand sighed, looking up at the stone above him. _'There's a way...but can I do it? Will it even work?'_ He shook his head before teleporting away, determined to find a quiet place to think.

* * *

Toriel looked out her window, staring at the place where the skeleton had just departed. Her face was warped with concern as she tapped her fingers against the counter. "Mommy?" She looked down, staring at her son's worried look. "Frisk will be okay right?"

"Of course Asriel," she said, patting his head, "We'll figure it all out. Now go play." He flashed a smile and ran off, calling for Frisk. She sighed, turning back to preparing lunch. _'I sure hope I'm right.'_

* * *

 **So there you have it. Hopefully you guys have enjoyed. I'm off. See you all next chapter.**


	4. Chapter 4

**Hey guys. Sorry this took so long. College work and YouTube caught my attention. Anyway, I've got a new chapter for you all. Sans' secret will finally be revealed. I hope you're all prepared.**

* * *

Sans watched as Frisk and Asriel played together, Frisk appearing to be telling Asriel some form of story while they made a snowman in one of the fields outside Snowdin. Alphys was with them, laughing along as they crafted their snowy sculptures. Sans watched on in silence, out of sight from the others. His left eye blazed, energy swirling around him. _'Heh, sorry kid. Hate ta use you as bait but...I need to meet with our little friend again.'_

"Hey punks!" Sans tensed as Undyne approached the small group, grinning sharply. He shook his head, focusing his attention back on Frisk. Undyne's appearance didn't sway his decision. After another moment of observation he let his energy fade fully. The small amount of energy he'd been keeping up to hide Frisk's presence faded away. Without pause he flicked his hand up, a wall of bones sealing the four other in.

Barely a second later a blast of energy slammed against his wall, and the beacon of Genocide Sans energy blipped into existence. "So," the other said, "You've decided ta keep protecting the kid huh?" Sans slipped back into sight, locking eyes with his genocide counterpart.

"You my friend are trying ta kill an innocent person," Sans said. His wall crumbled away, revealing the shocked faces of his friends, "now I dunno why you feel the need ta kill an innocent kid but I can tell ya one thing." His left eye flared to life, the right going completely black. Bright blue energy blazed around him, flickering and writhing like a living beast. "You better count on going through me first." The others eyes went dark.

"Oh boy," Genocide Sans said, "you're gonna have a bad time." Sans watched his double for a moment, taking a sharp breath.

 _'Oh boy. Here we go.'_ Energy blazed further, nearly engulfing him in blue flame. His doubles grin faded a bit, eye sockets widening a small bit. "You want a fight? Fine. Then I'm not holding anything back."

* * *

"Holy shit," Undyne hissed, staring at the pair of skeletons. Sans looked pissed, flames and energy spiking so much that it was starting to distort his features. "This is not going to end well."

"Undyne," Alphys said, "we have to get the children out of here." Undyne nodded.

"You go," the fish said, "someone should stay here and give this fool some back up."

"But Undyne-" the warrior cut her off, kneeling a bit to get eye level with her.

"I'll be fine Alphy. Now just take Frisk and Asriel and get out of here. I'll see you later." The other monster nodded, gathering up the kids and hurrying off. Undyne summoned her spear, turning back to the fight about to occur. Genocide sans was watching Sans without expression, and the latter was beginning to apparently shift. Undyne stepped back a bit as the sound of bones cracking echoed in the air, and a horrific roar ripped from the skeleton. Sans' face elongated slightly, skull becoming similar to that of his Gaster Blasters except sharper and more vicious. His ribcage and torso warped, stretching out and curving inward. Sharp, bone made spike crackled and jutted out of his spine, and limbs elongated. Teeth and fingers sharpened, a whip like tail of bone exploding outwards. The rippling screech that followed made the soldier cringe, staring at the flaming beast Sans had become. His jacket was torn, his shirt having shredded and fallen away due to his ribs expanding. His shorts barely hung on his pelvis anymore, his slippers having been destroyed in the transformation. _'What in the hell?'_ she thought, taking a step back. Sans roared, dropping on to all fours and snarling like a feral animal.

"So you brought out this side of you huh?" Genocide Sans said with a chuckle. His eyes flamed, bright, bloody red. "Well I wouldn't want you to get bonely. I guess I'll join you." Red flames blazed, and Genocide Sans entered a similar transformation. Once he'd finished, the other was nearly twice Sans' size, his clothes having shredded during the change. Sections of the bone appeared to be almost melting, less solid than the surrounding surface. "Do you honestly think you can beat me in this state?" Genocide Sans snarled. Sans roared back in response, tail whipping angrily. "Fine. I don't really want to kill myself but...if you insist on continuing this bone tired charade." Sans lunged at the other, front clawed limbs outstretched. Genocide Sans snagged him easily, slamming him down into the ground. Sans retaliated with a burst of energy, rolling away and bounding to his feet.

 _'Those two are going to tear each other apart at this rate,'_ Undyne thought, watching the pair hiss, and bite, and claw at one another. Where Genocide Sans was bigger, Sans was faster. However, the smaller skeleton seemed to be tiring, blue tongue lolling out of his mouth. Eventually the larger skeleton bowled him over, pushing his head into the dirt. Genocide Sans gave a wolfish grin, pushing all his weight onto Sans' chest and head. A sharp crack sounded, and Sans gave a screech of pain.

"Pathetic," Genocide Sans hissed, "you expect to protect them like this?"

"Back off!" Undyne shouted, throwing her spear. Genocide Sans jumped back as the energy blade slammed into his head and ricocheted off. Undyne summoned it back, falling into her fighting stance. Genocide Sans turned to face her, eyes flickering. "You wanna kill him!" she shouted, "Then you better count on going through me you bloodthirsty lunatic. I'm not letting you kill my friends! I don't care what happened to you. Doesn't mean you get to come to our time line and screw everything up!"

"Undyne..." Genocide Sans said, stepping back. Some form of sadness flickered in those red depths before the skeleton turned away. In the blink of an eye Genocide Sans flickered away, leaving the partially wounded Sans behind. Undyne waved her spear away, approaching Sans slowly.

"You alive?" she asked. Blue eyes slid open, and the morphed Sans got onto all fours and backed away with a snarl. The look in Sans' eyes was dangerous, like he was on the verge of attacking her.

"Frisk c-come back!" Undyne turned to see Frisk running towards them, eyes wide with worry. Alphys was trying to give chase, although Asriel was gaining on the human more than the elder monster was. Undyne reached out, grabbing Frisk's arm before the kid could run towards Sans further.

"Lemme go!" Frisk yelled, and Undyne actually released him in shock. Frisk never yelled or raised his voice. Instead he was usually soft spoken. Frisk stumbled forward, and Sans released a loud roar, crouching down. "Sans," Frisk said, so soft that Undyne barely heard him. The boy held open his arms. "I'm sorry you had to do this. You did it for me...didn't you." Sans gave a mournful sound, caught between a growl and a whine. Frisk smiled. "It's okay. You can turn back now. No one's gonna judge you." Sans gave one last cry, almost more like his normal voice than that of a beast.

And then he lunged forward.

* * *

 **And...cliff hanger. Hopefully you guys all enjoyed. I'm not sure when the next chapter will get out (hopefully soon) but that will be the chapter that explains why Sans has this transformation. The idea of this kind of Gaster Blaster Sans fascinated me when I found it, and I can't wait to implement it fully. So thanks for reading. Lemme know what you thought. I'll see you all next chapter.**


	5. Chapter 5

**Hey there everybody. Here's another chapter for you all. Things have gotten a bit stressful with finals approaching so updates are gonna be a bit sporadic. Sorry this chapter is a bit short, but hopefully you enjoy.**

* * *

Frisk gave a soft noise of complaint as the shifted Sans knocked him over. Sans snarled, looming over him. "Shh," Frisk soothed, running a hand on his snout. "I know you're hurt but it'll be okay." Sans gave a low rumble, stepping back with his head down. As Frisk stood, Undyne took a good look at Sans' injuries. The part of his skull near his left eye was covered in spidering cracks, and one of his ribs had been cracked and broken. The skeleton was crouched now, head hung in what the warrior could only assume was shame.

"I'll never understand it," Undyne said as Alphys finally reached her, Asriel at her side. "Frisk is able to befriend anything. So how is it that in the other timeline Frisk is a killer?"

"There are a lot of things that could have happened," Alphys said softly, "So we'll never know what happened." Undyne shook her head, looking back at the human. Frisk was gently patting Sans' head, the skeleton giving odd keening noises.

"Come on Sans," Frisk said, "You gotta turn back now." Asriel slipped away from Alphys, standing beside Frisk and holding his free hand.

"Yeah," the goat child said in agreement, "we want you to come back to us. Mom would be sad if anything bad happened." Sans' blue gaze flickered between them for a moment. Then, blue flames swirled around him. When they dissipated, the skeleton was back to normal. He was on his hands and knees, shaking slightly.

"We'd better hurry and get you back to Snowdin," Undyne said. Sans was trying to stand, but it was clear his injuries were paining him. Undyne stepped forward,bending and lifting the skeleton easily. He was fairly light, much to the Warriors surprise. Her shock faded to slight concern when he didn't even protest.

"I can...get us home," he choked out, voice strained. A hand went to the space where his rib was missing, hovering over the spot where it should be. A quick glance showed Alphys picking up the broken shard of bone from the grass.

"Don't strain yourself," Undyne said, "damned fool." Sans didn't reply. Instead he lifted his hand. His left eyes blazed blue and then, before anyone could react, they were back in the skeleton's house. He groaned, curling into himself and placing his hand over his left eye. The bone around it was cracking further, and his left eye socket was dark. Undyne set him on the couch,trying to pull his hand away from his wounded skull. "Alphys," the fish warrior said, "get me some water and a towel."

"O-On it!" The small monster ran off towards the kitchen. Undyne pulled Sans' hand away from his skull, eying the piece now missing entirely from his skull. The black hollow depth beyond it stared at her, not a glimmer in sight.

"UNDYNE? WHAT SEEMS TO BE THE MATTER?" The warrior jumped a bit, turning to look at him. Papyrus' eyes widened at the sight of his brother. "SANS?"

"Hey bro," Sans managed with a weak chuckle. "I uh...suppose you want an explanation huh." Papyrus approached him, sitting on the edge of the couch.

"YOU CAN EXPLAIN IT TO ME LATER," the skeleton said, pulling Sans into a position where he could hold him. "BUT FIRST I, THE GREAT PAPYRUS, SHALL HELP YOU." Sans chuckled, right eye sliding shut. The left didn't show any response. Papyrus shut his eyes for a moment, and when he opened them again the right one was blazing orange. Orange flames spiraled around them, bending to the younger skeletons will. Sans released a sharp hiss, and then the flames faded. When Papyrus' eye stopped glowing Undyne stared at the pair in awe. Sans was in one piece again, skull and rib looking as if they'd never been broken.

"Wow," Asriel said, "You never told us you could do that Papyrus."

"YES WELL...THATS ABOUT THE EXTENT OF MY ABILITY." Sans was seemingly asleep, and Papyrus set him on the couch as Alphys rejoined them.

"Thank g-goodness," she said, setting down the now unnecessary bucket in her hand. "I was so sure he w-wouldn't do it."

"Do what Alphy?" Undyne asked. The little monster looked away.

"I-I'm not sure t-that it's my p-place to say," she replied.

"Alphys," Undyne said, "This fool nearly got himself killed turning into some kind of giant skeleton beast. If you know why then enlighten us." Alphys shifted, uncomfortable.

"I j-just c-can't..." She said, "N-no one was s-supposed to know. I b-brought up the p-possibility but...I n-never thought he'd actually d-do it." Undyne looked ready to argue when a voice cut her off.

"Don't push her for the answer," Sans said, "she's only doing what I asked her to do." All gazes fell on Sans as he forced himself up. He looked shaky, as if one wrong move and he'd fall apart again. "When I did what I did, I expected that I'd have to explain myself. So just sit tight and let me tell my story."

"Sans," Alphys said, hanging her head, "I'm s-sorry I ever brought this u-up. I n-never wanted to make y-you relive the past."

"Nah," Sans said, "maybe it'll be good to get it off my ribcage you know? Just none of ya pity me ya hear me?" He chuckled, but the sound cut off into a pained hacking followed by a sharp groan. Frisk hopped up on the couch, simply a comforting presence.

"Seems we're gonna be here a while," Undyne said, "I'll make us something to drink." She wandered off into the kitchen, hearing Papyrus chastise his shorter brother.

 _'What the hell kind of past did this guy have in order to end up like this?'_ Undyne wondered, filling up a kettle to boil water for tea. _'And why doesn't Papyrus know about it? Those two grew up together so...how far did Sans go to keep whatever happened a secret?"_ She frowned, listening closely to the conversation happening out in the living room.

"Sorry Pap," Sans was saying, "I wish ya didn't have to hear this."

"I, THE GREAT PAPYRUS, CAN HANDLE ANYTHING YOU HAVE TO THROW AT ME. BRING IT ON!" Sans seemed to chuckle at that.

"Yeah I know," he said, "you're a tough dude." Undyne sighed, shaking her head.

 _'I guess I'll find out soon enough.'_

* * *

 **So there you have it. Next chap will be my little story on Sans' background. Hopefully you've enjoyed so far, and I'll get another chapter up as soon as I can. See you next time.**


	6. Chapter 6

**Hey there everybody. Sorry for the long wait. I'm back home from college now. Wh** **ich means I'm back to no internet as well. So updating will be a bit sporadic for the time being. Anyway, hopefully you enjoy this chapter everybody.**

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"So where do I even start?" Sans asked, sipping on the tea offered to him.

"How about starting with what the hell happened out there today?" Undyne commented. Alphys gaped at her.

"Undyne!" the monster said, "there are children here. Watch your language." The fish blushed, looking away. Sans sighed, pupils vanishing as he looked down at his cup.

"I guess I'll just start at the beginning then," Sans said, "Me and Pap here weren't born like every other monster." Papyrus was watching the other skeleton intently. "Pap...you remember Dr. Gaster?" Papyrus perked up at the name.

"OF COURSE I REMEMBER HIM!" Papyrus said, "HE WAS A GREAT SCIENTIST. JUST LIKE YOU!" Sans chuckled, looking away while a bluish tint flushed his cheekbones. Undyne blinked. She didn't even know he was capable of blushing.

"Yeah uh…about that. He wasn't as good a guy as you thought Pap." Sans shifted, looking extremely uncomfortable.

"I'M NOT SURE I UNDERSTAND," Papyrus said. Sans sighed, eyes sliding closed.

"Lemme just start at the start," Sans said, "It all started when Gaster, who was the royal scientist before Alphy here, got asked to make the DT machine."

"DT Machine?" Undyne asked.

"It was a machine to extract determin-nation," Alphys explained softly, "so we could harness t-the energy of human s-souls." Asriel shifted closer to Frisk, gripping his hand gently. "B-but it's powered down now!" She said hurriedly.

"Gaster found that particular project to be a bit…much for his old bones," Sans said, "and when he asked Asgore about getting an assistant the old fuzzball told him to figure something out. So…he made me."

"Hold up a second," Undyne said, holding her hands up, "he **_made_** you? How?"

"He carved out a chunk of bone from his left hand," Sans said, holding up the aforementioned limb, "Cultivated it in a test tube and manipulated the cells until they grew into a living being on their own." He flexed his fingers. "It wasn't that bad to start. He liked me, and I enjoyed the work. But…I never knew just what kind of predicament I was helping to start." He sighed, one hand covering his left eye as he slumped a bit. "But then…once the DT machine was basically complete…he decided I'd be more use to him and the rest of the Underground as a weapon. Course I wasn't exactly thrilled with the idea but wasn't ever gonna say anything. He'd created me, and back then that meant more than anything. He could've asked anything of me." His fingers clenched, eyes scrunching shut. "And oh boy did he ask a lot from me."

"S-SANS?" Papyrus said nervously. The taller skeleton sat beside him, and Sans seemed to relax. Even if just a bit.

"He did some simple experiments at first," Sans said, "things to help me control my magic. It was actually fun at first. Sometimes it felt like he saw me as his kid of something. Other times I was just data. But then…he wanted to do an experiment much more dangerous than any before." Sans looked up, pupils slipping back into sight. He looked at Papyrus, expression almost pained. "He didn't want to lose the progress he'd made with me…so he made you Pap."

"SANS…I DON'T EVER RECALL BEING EXPERIMENTED ON," Papyrus said, "AND CERTAINLY NOT BY GASTER. HE WAS OUR FATHER AFTER ALL."

"That's cause I made sure he never did," Sans said, "I convinced him that you were too young to handle the experiment. That, if something went wrong with me, he had the data to redo what he'd done with me already. It took a lot of convincing but he went for it."

"What was that experiment Sans?" Undyne asked. The smaller skeleton chuckled.

"Determination," Sans said, left eye blazing. Alphys jolted at the term, and Asriel squirmed uncomfortably. "He wanted to know if there was any way for a monster to contain determination in their bodies without degrading. And Paps…I barely knew you but you were my baby bones bro. I couldn't let you get destroyed like that."

"That makes n-no sense!" Alphys said, standing. Undyne shot her a surprised look.

"You didn't know?" Undyne asked.

"I d-didn't know the d-details," Alphys admitted, "just the end result." She looked at Sans, expression concerned. "But if you had determinat-tion injected into you…h-how are you still here?" Sans was silent, and the lizard-like monster leaned forward. "I've done that experiment, and seen the f-failures. It's n-not possible."

"You didn't have the skill Gaster had," Sans said, "and your subjects were already dead. Determination does funny things to a monster while they're still living."

"Go on," Undyne said, curious now that this conversation had started. She didn't know much about science like Sans or Alphys, but she knew about the Amalgamations that had been created from Alphys' work. And those bodies had been dead.

"Determination, if not handled correctly, makes the body break down," Sans explained, "If there's no soul to fight back, then the determination will take over and melt the body down into a barely sustained blob. If the monster is alive, however, and if their soul is strong enough, the determination can be contained. Determination bonds with the soul. In humans the soul easily contains determination. Frisk, whose very soul represents determination, is a perfect example of this." Frisk smiled, soul blipping into existence. "Monsters on the other hand would find it hard to contain that much determination. The breakdown we suffer is almost impossible to fight past. After all, feeling like your insides are melting and your whole body is turning to mush is a bit overwhelming."

"How'd you do it Sans?" Asriel asked, expression like a child who was extremely interested in a bedtime story.

"Honestly," Sans said with a shrug, "I'm not entirely sure. I was nearly gone when Gaster announced the experiment as a failure. But then…" He chuckled. "Then I just thought of little Pap, still sitting out in the living room playing and I knew in that moment that if Gaster thought I'd failed then his next plan of action would be to use him. Gave me the incentive to keep on pushing. Hell…I didn't even reform correctly. I became that beast you saw earlier. It's what inspired Gaster to make the Gaster Blasters to begin with. Gaster looked shocked, started wondering if he should use the DT machine on me. But…" The skeleton pulled up his shirt, revealing his soul to them. It beat in his ribcage, the white surface infected by deep red tendrils. "This kind of meld between the determination and my soul is the reason I survived that ordeal, and left me able to switch between forms."

"Speaking of that other form of yours," Undyne said, "why the size difference between you and that other Sans?"

"I have a theory," Sans said, pushing his shirt back into place, "that in order to beat the human in the other timeline, Genocide Sans injected himself with more determination."

"How do you figure?" Alphys asked.

"Not only is there a difference in size between our other forms," Sans replied, "his magic is red as well. That, combined with the fact that he appears to be bleeding despite the fact that me and Pap have no blood in our systems leads me to believe he used determination. Once the DT machine extracts it, it takes on a bright red coloration similar to fresh blood."

"That's not g-good," Alphys said. Sans nodded in agreement.

"I don't have a good shot against him," Sans said, "especially if I've got to worry about losing control and breaking down again. Only way I see myself winning that fight is if I can have more stamina. If I can push him far enough, he'll start to break down. Just can't be certain I won't fall apart with him." Sans jumped a bit when Undyne summoned her spear and pointed it at him. His left eye blazed to life, blue flame blazing around him for a moment before settling down. "I really wouldn't startle me," he warned, "I tend to get a bit violent when I'm nervous." The warrior ignored him, leveling a glare at his head.

"Get your ass up," she ordered, "we're going to train, and train hard. You need stamina? Fine. We're gonna do what we can so you can kick that other Sans' ass and get rid of him for good." Sans stood, putting his hands into the pockets of his jacket.

"I don't think that's a good idea," the skeleton said, "more likely to force an early breakdown if you get what I mean." Undyne grabbed his arm, dragging him towards the door. Sans only shrugged, and from the grin on his face and the way his energy calmed it was clear he was letting her do so.

"Papyrus, grab my armor," She ordered and the lanky skeleton ran off. She looked back at Sans, looking annoyed. "I don't care what you think. We're training."

"Your funeral," he commented before, absentmindedly, "who knows. Maybe it'll be fun. We could have a 'fun'eral rather than a funeral." Undyne shot him a look.

"For the love of-…shut up already. Just shut up." Alphys watched as she dragged him out, shaking her head.

"This…is not gonna end w-well…"

* * *

 **So there you go. Did you like? Leave me a comment and let me know what you think. Otherwise I don't know what you guys are thinking. Another story just finished up so this one will get a little more love for now. See you all next chapter everybody!**


	7. Chapter 7

**Hey there everyone. Sorry updating has been taking so long. I've been busy with everything else in my life. Anyway, I finally got this chapter finished. Hopefully you'll enjoy it as much as I do.**

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"Alright! Time to get this show on the road." Alphys sat off to one side, claws cenched in anticipation. Frisk and Asriel sat drawing to one side of her, yet Frisk was looking at her more than his paper. Papyrus was watching his brother and Undyne with rapt attention, and Alphys wondered if his attention was excited or worried.

"Uh…ya do realize that I could beat you normally right?" Sans asked, hands in the pockets of his jacket. Undyne scowled at him, pointing the tip of her spear his way.

"Watch yourself punk," she hissed, "Or else I really will beat the crap out of you. Now just do your little magic thingy and turn into that giant beast already." Alphys frowned at the twinge of regret that passed through Sans. It was small and barely noticeable, but it was there.

"Maybe we shouldn't do this," the skeleton said.

"Just shut up and turn." Sans shrugged, shutting his eyes gently. Blue aura blazed around him, engulfing him in flames. Bones cracked and shifted until Sans' energy calmed, his shift complete. He shook his head and blinked, tail whipping behind him. "Let's do this then. If you've gotta turn back then fine but otherwise we fight until we keel over ya hear me?!" Sans looked around, as if not entirely hearing her. She huffed, throwing her spear. It smacked against his skull, just above his eye, and he whipped back towards her with a roar. "Now we're talking." She summoned several spears, her original returning to her hand. "Time to see what you can do." Sans growled lowly, crouching like a feral animal. She fired of her spears, each one bouncing harmlessly off the skeleton. Then he roared, bounding forward.

"Undyne!" Alphys called. The warrior dodged his swipe easily enough, and the smaller monster forced herself to calm. Undyne could handle herself. Always had.

"Not quick enough," she mocked. He snarled, opening his maw. Energy collected and then fired, surging towards her. Undyne moved, narrowly avoiding the blow. Sans fired again, clipping her arm and shoulder. She stumbled back, armor warped and destroyed. "Well shit." She cracked the metal, trying to free her arm from the armor. "That was a good shot but uh…try to tone it down next time." Sans roared, charging another blast. Undyne stumbled back a bit. "Hey! Take it easy!" Her soul blinked into existence, flickering to a deep blue before being yanked out of the way of the oncoming blast.

"I GOTCHA UNDYNE!" Papyrus said and the warrior sighed.

"Thanks Pap," she said as he set her down. Sans turned to them, eyes blazing angrily. "Guess I pissed him off huh." Frisk slipped past her, jogging over to him. Sans swiped and Frisk hopped over the attack neatly. He patted Sans' snout, and the skeleton calmed. Blazing blue eyes reverted to white pupils, a low rumbled emanating from Sans' throat as he looked away abashed.

"It's not his stamina that needs training," Frisk said, running a hand over the skeleton's skull, "It's his self-control."

"I s-suppose it makes sense," Alphys said, wandering over, "Determination d-does do strange t-things to the m-mind." Sans gave a low purring noise in response.

"Guess we'll have to change up the training a bit then," Undyne commented with a frown, "You've got the strength, but that means nothing if you're going to act like a wild animal." Sans looked away, huffing. "Alright then. Let's get going."

* * *

Crimson eyes glared at the group assembled. _'No one can stand in my way now. This has to end. Now.'_ He shut his eyes, waiting as the change overtook him.

* * *

Sans' head snapped up, a feral snarl emanating from his lips. Red aura burst to life within the trees, Genocide Sans, pushing his way through them. Sans roared, blue aura blazing to life. "This is the last time we fight," Genocide Sans hissed, "You and me. One last fight with one victor." Sans tail lashed behind him, maw opening as energy built within. His double gave a half grin. "That's more like it."

"Move it!" Undyne shouted, scooping up Asriel and Frisk. The raced away as Sans fired, blast tearing up the ground before hitting Genocide Sans head on. The other skeleton snarled, bounding on top of the smaller skeleton with a roar. Claws flashed, scraping across the double's face. Bone cracked, red fluid spewing out. Genocide Sans screeched, baring his fangs. He pinned the smaller skeleton down, slamming his weight against Sans' ribcage.

"I will end you," he hissed. Sans growled, bending to sink his fangs into Genocide Sans' front leg. Liquid determination spewed free, and blue eyes met red.

"I…don't…think…so…," he ground out. The pair rolled over, Sans charging up a blast. Genocide Sans chuckled, determination unstabilizing him.

"Well…guess you're top skeleton this time," he said. Sans fired, the bluish-white energy engulfing both of them. When it faded both skeletons were back to normal. Genocide Sans was lying still, eyes closed and bones melting in some places. Sans sat beside him, panting lightly. His soul was buzzing with energy, red portions of his soul blazing like molten metal.

"Is it over?" Asriel asked.

"I think so kid," Sans said, standing shakily, "Let's get you home alright?"

* * *

"Oh dear…how pathetic." Genocide Sans forced his eyes open, blinking blearily in the half light. "Seems you've failed me…my little specimen."

"I only worked with you because we had a common goal," he choked out, "but ya know…I'm not so sure anymore." He looked up, catching the other's eye. "Dr. Gaster." Gaster grinned, expression nearly crazed.

"Not sure? That creature destroyed everything you held dear Sans, and you want to spare it?"

"Things are different here," Genocide Sans replied. Gaster knelt beside him, laying a hand on his ribcage. Before either of them could react Gaster thrust his hand past the bone, fingers squeezing around the other's soul. Genocide Sans gasped, eyes widening as Gaster drew back. Genocide Sans' soul pulsed in Gaster's hand, and the man smirked.

"You were so useful to me once," he said sweetly, toying with the soul in his hand, "but oh how you've changed. You've grown volatile. Like a dog snapping at its master."

"I knew trusting you was a bad idea," Genocide Sans replied, "Trust wasn't something you ever earned." Gaster looked down at him before standing fully.

"Trust? I never needed something like that from you. You were my creation, forged of my own flesh. No…I never wanted your trust. I wanted loyalty. A puppet who would eagerly follow my orders without question. But I see I've failed with you in that timeline. Time to wipe the slate clean and start again." Genocide Sans was quiet, watching him blankly. After a moment he spoke.

"What're you gonna do?" he asked.

"I created you," Gaster replied, "so I can end you without pause." Genocide Sans chuckled, shaking his head slightly.

"Nah I got that," he said, "I meant what're you gonna do with the kid?"

"Ah. That. I thought it would have been obvious." Gaster squeezed his fist, Genocide Sans' soul fighting to escape his grip. Eventually the energy from it faltered and began to fade. Genocide Sans coughed once, eyes shutting as his body disintegrated into white powder. A gust of wind blew, blowing the dust away. Gaster smiled, opening his palm to let the disintegrated soul scatter in the wind. "I **_will_** have that human child. And then…the experimentation will begin. Something about that child's soul makes it powerful enough to withstand enormous amounts of determination. And I **_will_** discover just what that is. Then…then I can be completed. Oh yes. Everything will slowly fall into place."

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 **So there you have it. Hopefully you enjoyed. I'm off, and I'll be back next chapter. Bye-bye for now.**


	8. Chapter 8

**Hey everybody. I've got a new chapter for you all. Hopefully you all enjoy. Lemme know what you all think.**

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 _Sans was standing in the center of destruction. Broken machinery littered the ground around him. Red liquid seeped from shattered bottles and beakers, making the room appear as if it was covered with a coating of blood. Sans was breathing hard, red warning lights flickering on and off in his vision. Yet for some reason he couldn't hear a thing. His soul was throbbing so much in his chest that it knocked against his ribcage a bit. His energy was flared around him menacingly, a figure slowly rising before him._

 _"Do you honestly believe you can win? That you can kill me with the pitiful amount of power you now hold?" Sans growled lowly, pupils blazing. "It's about time I ended you, and then the other one is next. I'll have to start over, but no matter. Perhaps this time I'll raise my subjects to actually obey instead of becoming rebellious little curs." Sans' anger spiked, the thought of Papyrus sitting upstairs completely unaware of what was happening down below making him see red. And not from the lights still blaring overhead._

 _"I will end you," Sans said. Sound suddenly came back to him, sirens blaring loudly. A deep rumble sounded the building's lack of stability. "You'll never get the chance to harm him."_

 _"We shall see." Sans swore he heard someone calling his name as his energy mounted and blackness overtook his vision._

* * *

"SANS!" The skeleton bolted awake, energy flaring. It searched out the three souls in the room, grabbing tightly and forcing them into the air. One of them cried out in shock, but Sans barely registered it. "IT'S JUST US BROTHER. CALM DOWN." Sans' mind finally registered his brother floating before him. Undyne was beside him, and Sans could hear someone, presumably Alphys, nearly hyperventilating behind him.

"Heh, sorry bro," he said setting them down, "Just a bad dream you know?" Papyrus nodded, but Undyne quickly made it clear she wasn't having any of it.

"Bad dream my ass," she hissed, "we all remember how your last dream ended. More of a god damn warning than a dream."

"No worry this time," he said, "because the person I dreamed of is certainly dead. I personally made sure of that."

"WHO WAS IT SANS?" The smaller skeleton looked away with sigh, pressing one hand to the temple on his skull.

"It was Gaster Pap," he said slowly. For a long moment Papyrus was silent. Sans shut his eyes. _'He probably hates me now. Gaster might have done some bad things but Pap basically idolized the guy."_ Papyrus was silent for another second before speaking.

"ALRIGHT." Sans blinked, looking back at his brother.

"You…aren't mad?" he questioned. Papyrus gave him a sad look.

"I NEVER KNEW OF HIS…ACTIVITIES, BUT I BELIEVE YOU SANS. IF YOU SAY HE DID SOME BAD THINGS, THEN I TRUST YOU DID THE RIGHT THING."

"Thanks bro," Sans said, "but yeah, Gaster isn't coming back Undyne. He's gone."

"Somehow I doubt that," She said.

"U-um…I might be able to help rectify t-the sit-tuation," Alphys said. Undyne crossed her arms.

"How so?" She asked. Alphys bit her lip.

"A-after you…killed Gaster," Alphys began, "his soul didn't fade, even t-though his body had b-been destroyed. Asgore t-tasked me with keeping an eye on it. It's safely locked aw-way in the lab." Before anyone could speak Sans' energy enveloped them all, teleporting them to the lab. Alphys looked slightly ill, while Undyne shouted at him angrily.

"Where is it?" Sans demanded, eyes scanning the lab.

"It's down b-below," Alphys managed. Sans teleported again as Alphys called for him to wait, reappearing in the room where the DT machine stood. It was deactivated now, but it still brought back memories the skeleton wasn't in the mood for. His energy spread out, searching for any trace of soul energy. The elevator beeped, signaling the descent of the others.

 _'Where have you been hiding?'_ He wondered, frantically seeking out Gaster's soul. Something caught his attention just as the elevator opened.

"Sans! Wait!" He ignored her, teleporting towards the source of energy he'd detected. His teleportation was interrupted, landing him outside of the room rather than inside of it. He frowned, trying again. His teleportation began before he was forced back to his starting position.

 _'Fine,'_ He thought, _'If I can't teleport in then I'll just have to force it open.'_ His left eye flared, a Gaster Blaster appearing beside him.

"Sans, stop!" He looked back, seeing Alphys rounding the corner.

"Sorry Alphy," he said, "I can't risk anything. Not this time."

* * *

 _'I've never seen him like this.'_ Undyne looked Sans over carefully. With his left eye blazing and the fur of his hood obscuring half of his face, she wasn't sure she'd ever seen him look more menacing. He turned back to the door before him, raising a hand. The Gaster Blaster opened its maw and fired, the blast slamming against the steel door. It groaned, buckling under the pressure. Sans moved, summoning up dozens of bones before sending them at the door. The combined force finally burned a hole through the metal, the edges glowing white hot.

Sans dismissed the Gaster Blaster, stepping through the hole. "What are you trying to prove?" Alphys called, "His soul **_is_** there. I'd never l-lie about that."

" ** _Was_** ," Sans corrected. His energy flared, nearly engulfing him entirely in flame.

"What?" Alphys said. Sans looked at him, eye sockets dark.

"His soul **_was_** here," Sans said, "It's gone now."

"That's impossible!" She ran forward, dodging around Sans and into the room. "H-how could it just be g-gone!"

"WHAT DOES THIS MEAN SANS?" Papyrus asked.

"It means Undyne was right," the skeleton replied, "It wasn't just a dream. It was a warning. A bad one."

* * *

Gaster smirked as he reappeared in the Ruins. His energy reached out, searching for the human's soul. _'Where could you be hiding?'_ The soul pulsated as his energy came in contact, shrinking away. _'There it is. Time to say hello.'_ He teleported again, ending in a child's bedroom. He sensed two souls; one human, one monster. He barely spared the monster child a glance before silently approaching the human. _'Human male, ten years of age. An extremely powerful soul full of beautiful determination.'_ He searched around in his mind, picking apart various bits of information. "So," he whispered, "your name is Frisk. Interesting." Dark eyes opened, eying him in confusion. "I need to ask you to come with me for a short time Frisk."

"I don't know you," Frisk murmured back. Gaster let his energy surround Frisk's soul, keeping all of its energy concealed.

"Don't you worry about a thing," he cooed, "I know Sans and Papyrus. I know them very well. In fact, I'm basically their father." Frisk's eyes widened, soul fighting against his hold. He tightened it, smothering any sign of the other's energy from outsiders. One last squeeze stopped its fighting, and Frisk's eyes slid shut. "Yes…just go to sleep now. You're going to be my next little project."

"Frisk…? What's going on?" Gaster turned around, eying the little monster child. "W-who are you?"

"That's no concern of yours," he said. His energy flared, attacking the monster's soul before he could even cry out. He fell unconscious almost immediately, slumping from the bed onto the floor. He felt another soul wake, panicked energy flaring before settling down slightly. He scooped up Frisk before glancing at the other child. "My, my," he said with a grin, "It appears I got more than I bargained for by coming here. How delightful." He flicked his wrist, lifting the monster child into the air. His energy searched his mind, pulling out a name. "Asriel…you were supposed to have died." He chuckled. "A human soul representing determination, and a monster who refused to die…it seems I have the perfect specimens for my rebirth." He hummed to himself, teleporting away to continue his work.

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 **So how was it? Hopefully you enjoyed, and I'll be back with a new chapter as soon as I can.**


	9. Chapter 9

**Hey there everybody. I've been having some writers block, but I came across an Undertale animation to the song ECHO by Zayn Inverse and that was my inspiration. Go check him out. Anyway, there likely won't be too much left to this story. Only another chapter or so I imagine. Thanks for all the support and please enjoy.**

 **EDIT** Sorry about the repeated text for anyone who saw. My technology hates me and I was on a time crunch**

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"So what's the plan here bone head?" Undyne asked, "You know Gaster better than anyone after all."

"You three grab the kids and Alphys," Sans said, left eye flaring with energy, "I'll take care of Gaster."

"You can't face him on your own," Toriel said, "His level of skill is unmatched."

"We'll see," the skeleton said. They reached the bottom of the stairs and Sans felt the icy presence of Gaster's soul. He let his energy surround him, letting the other skeleton know he was there. _'But where is he?'_ He wondered. They entered the main room, Toriel gasping and racing forward.

"My children!" She cried out. Asriel hiccuped, reaching out from behind the bars of his bone cage. Frisk lay on the ground beside him, breathing shallow and eyes closed. Sans focused on the energy, eye flickering between colors before the cage crumbled to dust. "My poor child. It's okay now." Sans watched as Undyne and Mettaton went to untie Alphys. Frisk opened his eyes gently, glancing at Sans as he got up.

"Go," he said, "All of you. Gaster is my problem, not yours." Undyne lifted Alphys into her arms, looking towards Sans with an odd expression.

"Good luck then," she said. Mettaton reached for Frisk as Toriel lifted Asriel, but the human pushed his hand away.

"I'm staying," he said stubbornly"

"Sorry kid," he said, "Determination won't be enough this time." He teleported them back up the stairs, just within his barrier.

"You should have let the kid stay," came Gaster's taunting voice, "You'll never win. Not against me." Sans looked at him evenly, aura flaring to its max.

"Maybe not," he admitted, summoning his Gaster Blasters to his side. The two beasts roared, charging up their energy blasts. "But I can't afford to let you free. At least if I go down then my power will lock you in." Gaster smiled, pupils gloowing brightly.

"I'll break free," he said, "I always do." Sans fired, the Gaster Blasters tearing through the floor. Gaster teleported away, reaching out a hand. He gripped Sans' soul with his power, yanking him forward. He flew across the room, hitting the floor and tumbling. "Pathetic," the skeleton said with a laugh, "just like these little weapons of yours." Black bursts of energy slammed into the Gaster Blasters, ripping them to pieces.

"No!" Sans shouted. Fragments clattered to the floor, and Gaster laughed. He listed the smaller skeleton again, throwing him towards the DT Machine. He caught himself, barely managing to right himself when Gaster appeared again. Energy tore into him, making him gasp at the shock as it knocked him back. _'Looks like I really have no choice other than to resort to that.'_ He let his energy envelop him, the determination in his soul taking over and changing him. Gaster watched him change without much comment. Sans shook his head, looking away.

"You've gotten stronger," Gaster commented. Sans lunged at him but Gaster was always slightly ahead of him. "However, I've changed since our last encounter as well. I've learned." Dark energy seized Sans, lifting him high into the air and squeezing his soul tightly. Sans felt his energy being suppressed, as if the flames his energy usually made were burning him alive. Slowly he changed back to normal, pain radiating through his body. Gaster threw him to the floor roughly. Sans pushed himself to his knees as Gaster knelt before him. He plunged a hand forward, grabbing Sans' soul and squeezing. "It's time to get back to where you belong."

* * *

"Ngah! This is so stupid!" Undyne stomped her way outside, crossing her arms. "Why the hell do we have to sit back and wait?"

"What else can we do?" Toriel asked.

"We could go back in there and help him!" She shouted.

"UNDYNE." They looked up as Papyrus approached. "WHERE'S SANS?"

"He stayed behind," Mettaton said, setting Frisk down.

"I'M SURE HE'LL BE FINE," Papyrus said, and yet they could all tell that he wasn't convinced by his own words. "HE'S THE STRONGEST MONSTER I KNOW AFTER ALL." They all stood silent for a moment until Frisk twisted his way free of Mettaton's grip on his hand and ran off towards the lab.

"Frisk!" Toriel called, "come back!"

"Hell yeah!" Undyne called out, summoning her spear, "kid has the right idea." She tore after him, Papyrus right behind her.

"Wait for me!" Asriel called, running after them. Mettaton followed, scooping the kid up before entering the lab.

"I can't believe this," Toriel said with a sigh. Alphys shrugged.

"if you c-can't beat them," She said, "join them."

* * *

"How much longer do you intend to resist me?" Gaster asked, "with your soul and the determination I collected from the child I can be whole again." He tugged hard, making Sans cry out. Gaster placed a hand under Sans' chin, tugging his head up. "Give in."

"ENOUGH!" A blast of orange energy slammed against Gaster's head. Sans' soul slipped from his hand, and the smaller skeleton gasped for air. "STEP AWAY FROM MY BROTHER."

"Pap," Sans choked out. Papyrus' right eye blazed orange, bones hovering around him. Undyne brandished her spear, Toriel summoning fireballs in her hands. Mettaton stepped before Alphys, crackling with electric energy. "I told you all to stay outside."

"Tough," Undyne said. Gaster's chuckle attracted their attention.

"You can't kill me in this form," he said, "I'm not fully in this world, thriving only on determination and what remained of my soul. Only one made of my own flesh could even hope to kill me. This one-" He lifted Sans into the air, flinging him at the nearest wall. Sans hit it with a sickening crack, tumbling to the ground. "-is broken. And my other child is useless." Papyrus deflated slightly, looking away. "Sans should have let me experiment on you way back then. Then maybe you'd be able to do something to stop me."

"Shut up," Sans murmured. He fired some bones at Gaster, the skeleton dodging easily. "Pap never needed to go through that." Frisk summoned his soul, and the red heart zipped toward Gaster.

"Do you think that will stop me?" Gaster asked. The soul surged past him, headed for Sans.

"Take it," the child said, "you need the traits that make up most souls. So I give you Determination." Sans grinned, and the soul's red color fused into him.

"Thanks kid," he said, getting to his feet, "but I don't think it'll be enough."

"I know," he said, looking at the others. One by one, minus Mettaton, their souls appeared; each was a different color.

"Heh I get it," Undyne said with a grin, "Then I pass on Bravery." The orange color drained out, surging towards Sans and fusing into him.

"I pass on Patience," Toriel said gently. The pale blue color made its way to Sans, mixing with the other colors. Asriel squeezed his mother's hand.

"I pass on Kindness," he said. The green color twisted and swirled before making its way to Sans.

"I p-pass on Perserverance," Alphys said. Violet color surged forward, spiraling a bit before mixing not in.

"Stop it!" Gaster roared.

"NO CHANCE," Papyrus said, "I PASS ON INTEGRITY." Dark blue energy raced to Sans, adding itself at last. Sans' soul pulsed, still missing one final component.

"You're out of luck," Gaster said, "there's nothing left." Sans stood, shutting his eyes.

"You're forgetting me G," he said, "the final piece, Justice, is mine." He opened his eyes, both blazing rings that flickered between rings of yellow and dark blue. Flames spiraled around him, making Gaster step back. "Now then, ready to have a bad time?"

* * *

 **So there you have it. Hopefully you all enjoyed. I'll be back with the next chapter as soon as I can be. Lemme know what you think.**


	10. Chapter 10

**Hey there everyone. This is going to be the second to last chapter for this story. Sorry it's such a short one. Hopefully you've been enjoying yourself so far and I apologize for the delayed chapter. Read on everyone.**

* * *

There was a moment of silence as Sans and Gaster eyed each other. Flames and shadows raged around the pair, energy oppressive. Gaster grinned, face and body distorting as Determination began to seep through. "Let's allow ourselves some privacy shall we?" Darkness flared around them, surrounding them in a black bubble. Sans' energy slammed against the barrier, failing to leave even a scratch on Gaster's energy.

"This will end here G," Sans said, "I can't let this go on any longer." Gaster's smile was warped and dark.

"We could have achieved something great together," Gaster hissed, "I could have made you into the perfect specimen with a little more work." Sans' eyes narrowed as Gaster laughed. "I could still make you perfect you know."

"You blew your chance G," Sans said, "and you lost everything."

"Did I now?" Gaster asked. "I still have a few tricks up my sleeve you see." Shadows spilled around his feet, creating seven shadowy figures with souls floating before him. Each soul was infected with deep strands of blackness. They corresponded to the traits he'd obtained earlier, and his soul gave a panicked hum as they solidified. "The essence pulled from human souls can be used in all sorts of ways, but do you know what happens when that essence is drained?" The green soul figure dove at him. In response his soul threw up a green barrier. The figure swung their weapon, smashing the barrier with ease. The second swing connected with his skull, knocking him backwards. "It creates the opposite of what was once their. You defend with Kindness, I'll strike with Brutality." Sans teleported away from the next strike.

 _'Alright then,'_ he thought, _'I can work this out.'_ He summoned his bones, letting them be imbued with energy. The pale blue of Patience colored them, and the weapons drove towards Gaster. The red soul shadow dove, slashing through the bones without difficulty. It turned to grin at him before lunging, shadowy knife raised. Kindness flickered to life, raising a shield. The shadow struck it, reeling back with a shriek. Gaster frowned. _'Okay,"_ he thought, _'trial and error. Green beats red.'_ He shot the other skeleton a grin, winking. "Looks like its gonna be check soon enough," he said.

"Do you think this game is so easily figured out?" Gaster questioned. He raised a hand. The orange shadow ran at him, the deep blue shadow following close behind. Sans dodged, throwing up barriers here and there to dissuade them. Eventually the dark blue shadow hit him, it's foot driving into his ribs. Pain lanced through him, making him stumble. The two shadows backed off, staring him down. "You boast on how Justice is your special trait, but it's easy to strike your weak spot isn't it." Sans stood, glaring. His soul was humming, flipping through colors rapidly. The Determination in his body pulsed, fighting to be free.

 _'There won't be much time if I do that,'_ He thought, clenching his jacket with one hand. The Determination pulsed and he shut his eyes. _'That's right. It isn't just my Determination I'm fighting with.'_ He opened his eyes with a burst of energy. His bones cracked and warped, bringing him into his beast like form once more. His soul knocked against his rib cage. The violet shadow fired off a stream of words at him, and his soul instantly reacted. Energy built in his throat, and he released a giant blast of energy. The pale blue energy struck the shadow and it screeched. The red shadow dove, slashing. It's strikes tore at the energy beam, and when it tapered off the violet shadow's soul was hovering a few feet above the ground. It shuddered and then exploded, dust sprinkling to the ground. _'Listen to your instincts,'_ A voice hissed in the back of his mind.

"Defeat one and more will take it's place," Gaster snarled. The yellow and orange shadows moved. The yellow one fired several energy blasts. He ducked around them, one smacking against his head before he shook it off. He snarled, opening his jaws as dark blue energy powered them. His jaws lashed through the yellow shadow with ease. "Watch yourself," Gaster taunted. Before Sans could react the orange shadow attacked, small arm slipping through his ribs to directly strike his soul. He jumped away, stinging slightly from the blow. A shout echoed through the shadowy dome, undeniably his brother's. "My puppets are attached to nothing. Yours are." He teleported away, snarling as the orange shadow continued to pursue him. He snarled, soul shifting. Violet overtook him as deep rings of energy exploded from his maw. The orange shadow dodged a few before being struck. He watched its soul burst before he was struck again. He swiped at the red shadow and it bounced away. Another cry ripped through the air.

 _'Alphys...'_ He huffed, forcing his soul to turn white again. Four shadows were left, and he knew he had to move carefully.

"All of this pain," Gaster said with a frown, "All because you simply won't give in. Tread lightly." The red, green, and dark blue shadows all lunged. He teleported away, trying to gauge them. His soul was fluttering through colors, unable to decide. Eventually it settled on yellow and he fired off several quick bullets of energy. The red shadow dodged easily, the dark blue one kicking each blast away. The green one reached him, slamming its weapon against his head. He opened his maw, slamming it with another blast of yellow energy. It shrieked, bursting. The dark blue shadow hit him, and the shock of pain nearly crippled him. The red went in afterwards, stabbing at his soul. He felt his energy wane, nearly throwing him back into his normal form.

 _'Just a bit longer,'_ he prayed. He teleported away to the pale blue shadow, letting his soul dictate his actions His tail glowed with red energy and he swung it, slashing the pale blue shadow in half. _'Two left.'_ He whipped around, focusing on the dark blue shadow. _'That one first.'_ His soul turned orange as he charged, His claws glowed with energy as he approached. The dark blue shadow swung a kick in defense but he ripped through it, wincing as the dust from its soul sprayed into his face. He glanced at Gaster, who was watching him with a passive expression. _'What is he up to...'_ he wondered just before the red shadow slammed into him.

"Do you know what the opposite of Determination is Sans?' Gaster asked. Sans raised a shield, watching as the shadow banged against it without care. Even as it's body began smoking. "It's Hate. A shock I know, but also fascinating at the same time. I would have assumed Hate was its own entity." Sans growled, crouching beneath his shield as it cracked. "How interesting that Hate can tear down anything in its way."

 _'Fine,'_ he thought, _'fight fire with fire.'_ He let the shield shatter, the shadow's blade catching his shoulder and sinking in. He roared, whipping his tail around as the shadow tried to free itself. The tip caught them in the chest, slamming all the way through. Their soul throbbed for a moment before shattering, the shadow melting away. Exhaustion struck him in waves, forcing him back to normal. _'No...not yet.'_ Gaster approached him, kneeling before his shaking form.

"Oh dear," Gaster said, "look at what you've done now." He grit his teeth as Gaster laughed. "how pitiful. I'll have to find a new specimen. I'm afraid you've finally worn down." The elder stood, turning away.

"I still have a bone to pick with you G," Sans said. Gaster turned back.

"Learn when to give in," the skeleton said.

"Sorry G," he said, "that ain't my style." A spear of red energy burst from the ground, piercing Gaster's stomach. The skeleton collapsed, only a few inches from him.

"I may have...miscalculated," Gaster murmured. He reached out, grabbing Sans' wrist with a maddening grin. "But I _**will**_ be reborn. That still remains true." Gaster faded into inky blackness, and Sans hurriedly shook the goo from his hand. A black ring was left singed into his right hand and he stared at it as the barrier faded. The colors fled from his soul, leaving it pale white once more. Even the red that once colored it no longer showed.

"I KNEW YOU'D DO IT BROTHER!" Sans grinned at Papyrus, sticking his right hand in his jacket as he got to his feet. Frisk smiled at him, and he ruffled the boy's hair.

"Let's get ourselves home yeah?" he asked, "we all gotta be tired after that." There was a murmur of agreement and he summoned the last of his energy, teleporting them all back to Snowdin. As they said their goodbyes Sans inwardly thought back to his hand.

 _'Hopefully it's just nothing.'_

* * *

 **So there you have it. The last chapter is the end for this story. However, there will be a sequel. It may take a little while to get up. What with YouTube, work, and college taking up too much of my time and all. Anyway, thanks for reading, and I'll see you in the next chapter.**


	11. Chapter 11

**Well...I might have lied a bit in my last chapter. I decided that instead of making a sequel, I'm just going to continue on this story. It doesn't seem like splitting the story in two will be a good thing at this point, so I'll just lump it all together. Hopefully you guys don't mind.**

 **Anyway, read on and enjoy.**

* * *

"SANS?!" Papyrus frowned when he received no answer from his brother. He huffed, looking up at his brother's door. Sans had been silent all day, and he'd had about enough of it. He took a single step up the stairs before taking a deep breath. He let his energy surge, teleporting himself outside of Sans' door. He congratulated himself for a moment before knocking on the door. "SANS? OPEN UP!" Again, no answer came. He grabbed the doorknob, twisting angrily.

The door opened without protest, and Papyrus blinked as Sans' empty room greeted him. He looked around, confused as to his brother's whereabouts. Eventually, after a bit of searching, he located a note, hastily scrawled on a stray piece of paper. He lifted the note, smoothing it out on the table.

 _(Dear Pap,)_

 _(I really wish I didn't have to do this to you, but in all honestly there's no other way to do this. So I'm just going to apologize for it) (now. I had to get away from Snowdin. Heck, away from all of it if I'm honest. That fight with Gaster didn't end, and I'm not too) (sure it ever will. I'm not gonna lie to you bro, cause I always swore I wouldn't. Things aren't good right now, especially not for me) (and I won't drag you in the middle of it. Gaster's still alive. And he's certainly not happy. So until I have a way to end him once) (and for all, you won't be seeing me around. I can't risk it. And if it comes down to it, I'm fully prepared to do what has to be in) (order to end it. So uh...see ya bro. Or maybe not. I can't really tell you for sure, but you'll be alright. Just...keep the kid safe...) (try not to worry to much would ya?)_

Papyrus stared at the note for a while, feeling his energy constrict his soul. What was his idiotic brother doing this time? He dropped the note onto the table, energy welling as tears in his eyes. _'I CANNOT ALLOW HIM TO DO SUCH A THING.'_ His magic took over, teleporting him out of the house. The snow felt frigid against his bones, and he shivered slightly. There was no trace of the elder skeleton. No sign of where he'd gone or how to find him. He hung his head, gritting his teeth.

 _'I NEED A PLAN.'_ He turned back towards the door, taking a deep breath. "UNDYNE! WE HAVE A PROBLEM!"

* * *

 _"How long are you going to continue running like this?"_ Sans ignored the voice in the back of his head, tugging his hood up further around his head. He'd had the river person bring him far north in the underground, to a barren space where no monsters really inhabited. A completely barren area would be impossible to find he knew, so one that was mostly empty would have to do. " _I'll break free you know. It's just a matter of when."_

"Shut up," he growled lowly. Gaster fell silent, and the skeleton sighed. He continued to walk, tense and ready for a fight at any moment. His preparation was a good thing it turned out. Gaster lashed out, energy ripping against his own. He faltered, nearly stumbling. _'He's getting stronger. This isn't good.'_ He sighed, trying to summon a Gaster Blaster. He had to end it now, before Gaster got any stronger. Yet the Gaster Blaster that appeared was not under his control. Instead of the bright blue eyes he was used to they were pale white, cracks littering its surface. He stepped away from it as Gaster's laughter echoed in his skull.

 _"Stand down my creation,"_ Gaster said, _"Your time is up."_ Sans felt the tug on his soul, the dark energy that engulfed him. He cried out, trying to fight back. Yet before he knew it, he was locked within his own body. A dark barrier let him see what his body saw, and yet no matter how hard he tried, he couldn't even scratch it. "Finally," Gaster said in a warped version of Sans' voice, "Now I will be able to finally finish what I started. The Underground will be eradicated of fools, and I can continue my research with new vigor."

 _'I have to warn them,'_ Sans thought, _'but how? I can't go anywhere...'_ He growled under his breath, punching the barrier of Gaster's energy. The elder skeleton chuckled, golden energy blazed around him. _'No!'_ The energy faltered, but only barely.

"Settle down Sans," he said, "I'd hate to have to kill you over something so trivial." Golden energy overtook them, and Sans found himself looking out at his bedroom. The note he'd left Papyrus lay forgotten on the floor, and he tried to find his brother. He nearly sighed in relief when he found no trace of him. "Too bad," Gaster said, "I was hoping to clean up two messes today." Sans remained silent, fuming internally.

 _'Although this does raise a good question,'_ Sans thought, _'Where **is** Pap?'_

* * *

"He what?!" Undyne shouted. Papyrus had gathered them all together at Toriel's home in the Ruins before telling them the news. Undyne was certainly not happy. "He's such an idiot! How can he expect to solve this on his own?!"

"HE'S ALWAYS BEEN LIKE THIS UNDYNE," Papyrus replied, "HE NEVER TRUSTED HIS PROBLEMS TO OTHERS, AND THEREFORE ALWAYS TOOK THEM ON ON HIS OWN." Undyne fell silent, still fuming. Toriel sighed, shaking her head.

"This is certainly troubling," She said, "we have to find him."

"H-how are we g-gonna deal with G-Gaster?" Alphys asked, "h-he's strong. I d-don't think even Sans c-can contain him for long." They all fell silent, thinking. Asriel shifted, leaning over to whisper in Frisk's ear. The human cocked his head, listening intently before nodding.

"We have a plan!" Asriel shouted, hopping up off his mother's lap. Frisk followed, smiling. "We just gotta go see Dad!"

"No offense kid but Asgore's kind of a pushover," Undyne commented. Asriel barely seemed to hear her.

"Come one guys!" He said, half running for the door. Mettaton caught him easily.

"Anyway you plan on informing us of your plan?" The robot asked, settling the goat-monster on his shoulders. Asriel shook his head.

"Nope," he said, "but we gotta hurry."

"Well it's not like we have a better plan," Undyne said, "So let's get going." Papyrus let his energy engulf them, attempting to teleport them. After a moment they vanished, leaving the house empty.

* * *

"Well well," Gaster said, looking around the quaint little home. "The queen has certainly changed her living style." Papyrus' energy permitted the air, like a sort of beacon. Gaster grinned, feeling Sans' soul within him grow restless. _'Time for the game to begin it seems.'_

* * *

 **Well there you have it guys. Honestly this added section won't take too long. Only like two chapters or so (hence why I added it on here). Also, Google Gaster! Sans to see what kinda look I'm going for with Gaster atm. Basically I figure that's a great mix for Gaster and Sans, which is what's happening here. Anyway, until next time everyone.**


	12. Chapter 12

**_Hey guys. There should only be...two more chapters after this? I'm just approximating, but two or three sounds about right. Hopefully you guys are enjoying so far and please review to let me know what you think._**

 ** _Well, read on everyone._**

* * *

"Well we also most made it." Papyrus hung his head, disappointed he'd only been able to teleport them as far as Hotland. They'd been forced to trek the remaining distance to Asgore's castle, and now the front gate loomed before them. Asriel broke from his mother's hold, running towards the gate.

"Dad!" He called out, knocking loudly, "Dad, open up!" After a moment the door opened, and Asgore received a handful of his son.

"Well howdy Asriel," he said. He looked up, smiling at Frisk. "I see you're here as well Frisk." The human grinned, running to the elder monster. Asgore lifted him easily, holding one child in each arm. He looked up at the other monsters with a frown. "What seems to be the problem."

"Gaster," Undyne said. Asgore's expression darkened.

"How?" He asked.

"He broke free," Undyne said, "Sans thought he'd stopped him but it seems Gaster didn't go down quite yet."

"We thought you might be able to help dad!" Asriel said. Asgore chuckled, looking at his son fondly.

"Anything I can go to help," Asgore replied, "but I'll need a better idea of what I **_can_** do first." Asriel frowned, thinking deeply. It was Frisk who moved first, his soul blinking into existence. The soul was glowing brightly, crimson spilling across the floor.

"Determination's likely to make it worse," Undyne commented, "That's what's keeping Gaster alive after all."

"Determination may have been the answer in the past my child," Toriel said, going to take the boy from Asgore, "but I'm afraid this time it won't be enough. Besides, I cannot allow you to fight in this."

"YOU'VE DONE PLENTY HUMAN," Papyrus said with a grin, "NOW IT'S OUR TURN TO FIX THINGS."

"D-Don't worry Frisk," Alphys said, "We'll figure something out." Frisk sighed, looking away for a moment. Then he squirmed his way free of Toriel's grasp, jogging off into the castle.

"Poor Frisk," Mettaton said, "The kid's used to always having the answer."

"That's what worries me," Toriel said with a frown. Asriel twisted free of his father's grip, racing off.

"I'm gonna go find Frisk!" He called back. Toriel sighed, shaking her head.

"If only those children could have a normal life," she murmured, "Then I could finally relax." Asgore approached her, wrapping on arm around her shoulders. To his pleasant surprise she didn't shake him off.

"You and me both," Asgore said softly.

* * *

"Frisk!" Asriel called out, trotting down the corridor. The halls were silent, no sign of his younger sibling. "Frisk? Where are you?" The monster child frowned, searching for the other. As he wandered deeper into the castle, worry began to settle on him. "Come on Frisk. Don't hide from me."

"Asriel." He jumped, whipping around. Frisk stood just a few feet behind him, an unreadable expression on his face. Asriel sighed, holding one hand to his chest.

"You scared me Frisk," the monster child said. Frisk gave him a small smile.

"I'm sorry," he said. Asriel grinned.

"It's okay," he said, "I came to find you anyway. Let's head back." He took Frisk's hand, tugging him back the way he'd come. When Frisk didn't budge Asriel looked back at him. Frisk frowned, gently pulling his hand away and shaking his head. "Frisk?" Asriel questioned, "Is something wrong?"

"I can't leave yet," the human child said softly. His eyes shone with guilt and apologies, focusing solely on the hard floor. "I still have something I have to do." Asriel gave his sibling a curious glance before understanding dawned on him.

"You have a plan?" he asked. Frisk nodded. "That's great! Then we should go tell mom and dad. They'll help us and we can go get Sans back." He grabbed Frisk's wrist again, and again the human child pulled away.

"This is..." he trailed off, despair settling on his features, "They won't approve Asriel," Frisk finally said. The monster child took a small step back.

"What do you mean?" Asriel asked. He had a small flashback in his mind,back to a time when Chara had something very similar. They'd nearly killed his father that day. "Frisk...what are you gonna do?"

"Undyne was right," Frisk said, stuffing his hands in his pockets, "Determination won't help...but I think I know what might."

"I don't wanna hurt anyone," Asriel said softly. Frisk shook his head, giving the other a sad smile.

"Neither do I," Frisk replied. Asriel relaxed a bit, taking Frisk's hand.

"Don't worry Frisk," he said, "Whatever it is, we'll do it together. I believe in you."

"Thanks,"Frisk said. He squeezed Asriel's hand. "but I don't want you to get hurt if you come with me."

"Why would I get hurt?" Asriel asked. Frisk leaned towards him, whispering the answer as if the walls could hear them. Asriel's green eyes widened, but the monster child didn't pull away.

"I'll be okay," he said, "besides, I'd be more hurt if you just took off on your own." Frisk smiled, soul blinking into existance. Asriel's followed, the pair of souls entwining their energy. "Now c'mon," Asriel said, determination in his voice, "Let's go do this."

* * *

"The kids sure have been gone a while," Undyne commented. The group had been trying to think up some form of plan, but the lack of Frisk or Asriel began to set them on edge.

"It's beginning to worry me as well," Toriel replied.

"I can go take a look around for them if you'd like," Mettaton offered, "I'm not much use here after all."

"Thank you Mettaton," Toriel said, "It would put my mind at ease." The robot nodded, standing and turning away. Before he could leave Alphys jumped to her feet with a gasp.

"O-Oh no!" she shouted. Those around her stood as well, a mixture of shock and confusion on their faces.

"What happened?" Undyne asked, spear held ready to defend. Alphys shifted for a moment, anxiety coming from her in waves.

"I-I think I k-know where F-Frisk went!" she replied, "f-follow m-me!" She ran off, the rest of them following quickly. The small monster knew exactly where she was going, heading deeper into the castle before reaching a staircase descending down into blackness. Asgore tensed, knowing exactly where the smaller monster was bringing them.

"Why would Frisk come down here?" he asked, aware of the likely answer but needing her to disagree.

"Every aspect of the soul has an opposite," Alphys said, "Even D-Determination." She hurried down the stairs, lights illuminating as she went. The group rounded the corner, the abandoned coffins that once held the human souls sitting before them. Asriel was on the floor, gaping at the human before him in an expression that was half terror and half awe.

"Thank goodness my children," Toriel said, "You're both safe." She went towards them, but Asgore stopped her with an arm.

"We're t-too late," Alphys said, slumping. Frisk turned...although the face that greeted them was no longer Frisk's. Crimson eyes blazed, chestnut colored hair framing their face. But what really unnerved the group watching was the twisted, sinister smile painted on the human's face. Toriel stepped back,eyes widening.

"Don't be mad," Asriel practically begged, eyes flickering between his parents. No one answered him, all eyes on the human.

"How can you be alive again," Asgore asked breathlessly, "Chara..." She smiled.

"The answer to that is simple," Chara said, "I'm alive because of Determination."

* * *

 **So there you have it everyone. Hopefully you enjoyed the chapter. I'm off to write other fics but I'll be back soon enough. Bye-bye everyone.**


	13. Chapter 13

**Hello everybody. I'm finally back. I've had writer's block on this piece lately, but it's good now. We're getting to the end here, so hopefully you'll all enjoy this chapter.**

* * *

"Release Frisk," Asgore said. He and Undyne took up defensive positions, both pointing their weapons towards Chara. The human cocked her head, laughing. Asriel gripped her arm tightly.

"Release him?" she mocked, "but he gave himself to me. We made a deal, one I enjoy very much." Asgore looked skeptical, but lowered his trident a bit.

"What deal?" he asked. Chara grinned, eyes glittering.

"Frisk and Asriel approached me," she said, "and Frisk asked for my assistance. You see, I'm tired of being stuck in this prison. Frisk agreed to not only let me reside within his soul, but also gave me control. At least until I finished helping him."

"Helping how?" Undyne growled, glaring. Chara giggled.

"Isn't it obvious?" she asked, "I heard you've been having a bit of trouble with a certain skeleton going rogue?" Their group tensed, and Asriel shrank down even more, looking almost ashamed. "I must admit, I've been looking forward to a rematch with that one."

"WHY?" Papyrus asked, "SANS NEVER FOUGHT YOU!" Chara sighed, still smiling.

"Maybe not in this timeline," she said, "but he has. Killing him was fun, and I'd very much love to do it again. Even if it isn't really him I'm fighting."

"F-Frisk said that Chara would be able to help," Asriel said, "something about her soul counteracting Gaster's." Asgore glanced at Alphys. The monster was shifting uncomfortably.

"Is this true Alphys?" he asked, "Could Chara actually fight Gaster?" Alphys sighed, looking away.

"Y-Yes," she said softly, "S-She might be the only o-one who c-can." They all fell silent for a while, Papyrus breaking the silence.

"YET AGAIN FRISK HAS THE ANSWER," he said. Toriel shook her head, tears gathering in her eyes.

"Why would you lead Frisk here?" She asked her son. Asriel frowned.

"I thought I could help," he said softly.

"You could've gotten yourselves killed!" Toriel shouted. Asriel flinched, looking at his feet. Asgore laid a hand on Toriel''s shoulder, calming her.

"He meant no harm," Asgore said, "Besides, Frisk knew Chara's soul was down here already. I warned him not to come here."

"That's right," Chara piped in, "Frisk came looking for me. Asriel just stumbled into the mess. You always were quick to point fingers at the wrong people mother." Toriel sighed, looking worn down. "Now then, I'd like to keep my end of the deal. After all, when I make a promise, I always keep it." She hummed, looking around. "Now then. I just need a weapon." She wandered off, Asriel chasing after her. The group shared looks of uncertainty."

"Should we allow her this freedom?" Toriel asked.

"I'm afraid we have no choice," Asgore said, "Gaster has combined with Sans to create a being so powerful I don't know how we could ever hope to defeat it alone." He glanced in the direction Chara had gone, a frown deeply etched into his face. "However, I don't think Chara is necessarily the solution."

"I-I'll work on a backup p-plan," Alphys said immediately, "Und-dyne, can you help me?"

"Uh sure," the soldier replied, "but I'm not exactly good at science stuff. You know that." Alphys smiled gently.

"M-More like I need help m-moving some stuff," she replied. Undyne flashed a grin.

"Yeah okay," she said, "That I can handle."

"YOU'D BEST HURRY," Papyrus said, looking worried all of a sudden.

"How come?" Undyne asked.

"SANS...I CAN SENSE HIS ENERGY. HE'S HERE." They froze, letting his words sink in. Alphys ran off, Undyne following a moment later.

"We'd best go upstairs," Asgore said, setting a hand on Toriel's back, "At least then we can keep an eye on the children if something goes wrong." Toriel nodded, and they headed upstairs in silence.

* * *

"About time you showed up." Gaster paused, surprised to hear a voice. He turned, spotting the human child before him. The other child, Asriel Gaster reminded himself, stood with them.

 _'This Determination. with such a concentrated amount, I can finally complete this form and be strong once more.'_ Sans' soul flared within him. He swatted the energy down, nearly extinguishing it. _'And then, I'll be able to keep this one quiet.'_ He summoned his energy, yellow flames licking around him. _'This should be easy enough.'_ He lunged for them, intending to knock them out easily. The human dropped, lashing upwards. A blade nicked the bone in his arm, and he hissed as Determination bled from the wound.

"Did you really think it'd be so easy?" the child mocked, grin twisted. Sans' soul shuddered from within him.

"Who are you?" Gaster asked. _'This child...is not the one I saw before.'_ They smiled, fingers twirling the knife in their hand.

"And here I thought you'd recognize me," they said, "although you never approved of me did you?" The recognition struck him, and his eyes narrowed.

"Chara," he said, "and here I thought you were dead." She giggled.

"I used to be," she said, "but I've made a deal. So I'd suggest you release the comedian. If you don't, I'll just have to beat you out of him. Personally, I like the second option."

 _'They must have been desperate,'_ he thought, _'If they brought her back.'_

 _"What can I say G,"_ Came Sans' voice from within, _"They're determined."_ Gaster frowned.

 _'No matter,'_ he thought, _'They can't beat me. No matter how much Determination they have within them.'_ He summoned a Gaster Blaster, the beast roaring.

"So that's how you wanna play," she said with a laugh. She raised her knife, grinning wildly. "Good. I like a good fight." She lunged for him, narrowly evading the Gaster Blaster's strike before slashing. He teleported away, hissing at the cut scoring his chest.

 _'She's powerful,'_ he thought. She smirked, as if hearing his thoughts. _'but no matter. I'll take her down, and extract what i need from her soul.'_ Chara grinned, raising her blade. He summoned his blaster, and Chara cocked her head.

"Do you really think that can kill me?" Chara asked mockingly, "I've fought stronger than you, and won." He scowled, the blaster's eyes glowing red. The blast of energy bore down on the child, enveloping them.

 _'It seems that I'll need to find another way to get Determination,'_ he thought with a sigh, turning away from the blaster as it continued to fire.

* * *

 _'No...kid!'_ Sans slammed a hand against the walls of his soul, staring in horror as the Gaster Blaster attacked Frisk and Chara's fused form. Gaster dismissed it, but his eyes watched on. _'No way...they can't be dead...'_ He squeezed his eyes shut, slumping to the ground. _'It's over.'_

A giggle broke through the air, drawing Gaster and Sans' attention The energy from the Gaster Blaster dissipated, and a black slash sliced the weapon in half. "Don't you know that every element of a human soul has its opposite?" Chara asked in a mocking tone, "And do you know what the opposite of Determination is?"

 _'She's invulnerable to Determination,'_ Sans realized. Chara's soul, or perhaps it was Frisk's soul, flickered into existence. The red heart was ingrained with black tendrils. Gaster stepped back, frowning, and Sans noticed something. There, seated within the red heart, was Frisk. The human child smiled, pressing his hands against the soul. Sans' eyes narrowed. _'I've gotta get out of here. Frisk can only keep Chara in check for so long.'_

"You represent Hate," Gaster said. He laughed, grinning wildly. "Good. This should make things much more interesting." Chara smiled.

"Yes," she said, "it should makes things _**much**_ more interesting."

* * *

 **So there you have it. Hopefully you guys all enjoyed. Leave a comment to let me know how it's going. Bye-bye for now.**


	14. Chapter 14

**Hey guys. Sorry this took so long. I kind of lost motivation for a while but it's back now! Hopefully you enjoy the chapter. I personally love how this went. Wrote something a little special that I really wanted to include. Hope you enjoy because this is the last chapter. Thanks for joining me on this ride.**

* * *

Sans watched as Chara and Gaster fought with deadly grace. However, he eventually tuned it out. He needed to escape so he could help Frisk. He could feel the souls of his friends approaching. Perhaps if he got free and gathered their energy again...maybe he could do something.

His struggles proved useless. Gaster's hold on his soul proved more powerful than his weakened state could handle and he fell back panting. The others would be there soon, and he wanted to avoid involving them more than necessary. Again he saw Frisk, looking in his direction with a sad expression. His power flared, but Gaster smothered it easily. All it managed to accomplish was causing Gaster to take a swift blow from Chara before she darted away again.

"Stand still you little brat!" Gaster hissed, summoning his Gaster Blaster. Chara giggled, darting forward just as it blasted. The collision between Gaster's Determination and Chara's Hate send a shock-wave through the room. It set his senses on the fritz, making it nearly impossible to sense his friends underneath the power. They seemed stuck on the other side of the door.

 _'Good,'_ he thought, _'Less people to get hurt.'_

"Sans!" He started, grateful that Gaster turned to look at whoever had called his name. He placed his hands against his soul's inner wall, staring at Alphys as she appeared at the top of the stairs, looking out of breath with a small device in her hand. She froze, staring at Gaster, eyes wide.

"Alphys," He greeted, "It's been quite a while." Alphys swallowed, taking a deep breath.

"S-Sans!" She called again, "I'm g-gonna get you out o-okay?" He focused on her, watching as she threw the device in her hand. Gaster, likely on instinct, caught the tiny device. It beeped, lighting up with a blinding blue light. Gaster released it, but the device latched onto their shared soul, cracking it.

"Go now!" Alphys called out. Sans grit his teeth, forcing his way free. He was non-corporeal, floating freely, and whether or not anyone could see him was still a question. Gaster roared in anger, and he turned to look back at the elder skeleton. Gaster's form seemed to be falling apart a bit, the skeleton unable to hold himself together entirely without Sans' energy. Alphys gave a shout of success, looking amazed.

"No matter," Gaster growled as the chaos between him and Chara began to die down. "You were holding me back anyhow. This way you won't be a nuisance."

 _'Now what?'_ Sans wondered with a frown. His answer came in the form of Chara shrieking in a tone between pain and shock. Sans spotted Frisk smile at him before A rush of crimson energy burst from Chara and Frisk's combined soul. Frisk's spirit hover by him before flooding into his soul, returning him to a solid state. _'Thanks kid,'_ he thought, eyeing both Chara and Gaster, _'although it might not be enough.'_ Frisk's giggle sounded through his mind.

 _"This isn't all we can do together Sans,"_ Frisk said. Before he could question what that meant his soul flickered into existence, rapidly flipping through colors. His body shifted, as if he were changing into his Gaster Blaster form. Yet his bones didn't feel like they were cracking quite as violently. When he finally opened his eyes and glanced down at himself he grinned.

"Oh hell yeah," he said, looking up at the shocked expressions on the people in the room, "You've outdone yourself this time kid." He felt the happiness radiating from the human sharing his soul and tried to summon his power. Blue flames hovered above his right hand. "Alright," he said, "it's time for you two to have a bad time."

* * *

"God damn door!" Undyne shouted, growling at the door blocking them from the room where they could hear a fight raging. She kicked it, but it held firm. "We'll never get in at this rate!"

"STEP BACK." Undyne blinked at the order, staring at the skeleton. Papyrus' right eye was glowing orange, his hand held out before him. His face was contorted with concentration, energy gathering slowly before him.

"Uh Pap?" Undyne began, "What're you do-"

"STAND BACK UNDYNE!" She jumped back, a bit startled by the urgency in his voice. Just as she did the energy before the skeleton formed into a Gaster Blaster similar to Sans'. This one was much larger, although shorter with glowing orange eyes. Papyrus hissed as the beast opened its maw, energy building within. It fired, ripping the door off of its hinges. The weapon immediately disintegrated, and Papyrus was left looking winded.

"Well that's one way to do it," she commented. She dashed in, staring at the scene before her as the others filed in shortly after. Frisk stood in the center of the room, wearing Sans' clothes and with a Gaster Blaster by his side. Chara was floating nearby, looking angry and glitching out every so often. Gaster looked displeased as well, "Hey punk!" When the human turned around Undyne realized the changes immediately. The human's eyes were open, bright blue irises staring at her. They had a lopsided grin on their face, hands stuffed in their pockets. Blue flames were flickering around their left eye, and Undyne stepped back. "What in the world is that?"

"A soul fusion." Alphys pressed against her side suddenly, staring as Frisk turned back towards the group. "Although I believe Sans is in control."

"The hell does that mean?" She asked.

"It means Sans and Frisk have fused together in perfect sync," Alphys said slowly, "It's...not something I thought scientifically possible. Especially between a human and a monster. The form is extremely powerful but also extremely unstable."

"Now what?" Undyne asked. Alphys smiled at her.

"Now we let them take care of this."

* * *

"Now which one of you wants to get dunked first?" Sans asked. Chara screeched, surging at them. Her black soul burst into existence, giving off a feeling of dread. The Gaster Blaster moved before them, rapidly morphing. It became smaller, much slimmer than the previous Gaster Blaster that once stood in its place. It blasted a burst of white energy, flooding the area with light. Chara screamed as the light enveloped her. When it faded, the human was nowhere in sight. The Gaster Blaster reverted back to the one Sans normally used, roaring angrily. "Alright G," Sans said, "You're next."

"Do you believe you can beat me with whatever new power you now possess?" he challenged. Sans grinned.

"Hell yeah," He said, patting the Gaster Blaster on the head, "cause this guy here just got an upgrade."

"Upgrade?" Gaster questioned. Sans winked as the weapon shifted again. It elongated, becoming sharper along the edges as it's color darkened to black. The eyes turned blood red and it screeched angrily.

"This guy changes attributes," Sans said simply, "Into exactly what's needed to end you." Gaster looked put off, but said nothing. "Now then. Time to end this." Gaster scowled, his own Gaster Blaster fired. Sans teleported, landing on his weapon's head. It charged up, black energy surging. Gaster avoided it easily, but the blast left a pile of black energy on the wall and ground. It reached out when Gaster reappeared, trying to grab at him before the skeleton backed off. "Hate sure is a pest isn't it? It just grabs at everything around it, taking over."

"It won't work Sans,"Gaster growled. The Gaster Blaster fired again, shattering Gaster's own weapon before turning back to him.

"Just give up G," Sans said lowly, "Or else you really won't like what happens next." Gaster scowled, charging at him as Determination fired off at him in little blasts. They bounced off the altered Gaster Blaster harmlessly, and it opened its mouth to intercept the other. Sans hopped down, watching as Gaster struggled in the weapon's jaws. "One more chance G," Sans said, "Just give up quietly."

"Never," the former scientist sneered. Sans shrugged.

"Suit yourself." The Gaster Blaster bit down, black energy vaporizing the skeleton until a small, melting soul sat before him. Sans scooped it up, encasing it in his own energy before finally snuffing it out. "I warned ya G," he said softly. Frisk separated from him, hugging him tightly. The human looked up at him, an unspoken question hanging between them. He smirked, swinging the child up onto his shoulders. "Yeah kid," he said, "It's all over." He turned to the group as they ran to him, asking a thousand questions. He only half listened, finally content for the first time in a long while.

* * *

 **There you have it guys. Hope you enjoyed. That's all I've got right now. Maybe I'll do more Undertale fics later on but not sure for now. Let me know what you thought and I'll see you guys in another fic. Bye-bye everyone!**


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